In order for the human eye to have clear vision of objects at different distances, the effective focal length of the eye must be adjusted to keep the image of the object focused as sharply as possible on the retina. This change in effective focal length is known as accommodation and is accomplished in the eye by varying the shape of the crystalline lens. Generally, in the unaccommodated emmetropic eye, the curvature of the lens is such that distant objects are sharply imaged on the retina. In the unaccommodated eye, near images are not focused sharply on the retina because the focal points of their images lie behind the retinal surface. In order to visualize a near object clearly, the curvature of the crystalline lens must be increased, thereby increasing its refractive power and causing the focal point of the near object to fall on the retina.
The change in shape of the crystalline lens is accomplished by the action of certain muscles and structures within the eyeball or globe of the eye. The lens is located in the forward part of the eye, immediately behind the pupil. It has the shape of a biconvex optical lens, i.e., it has a generally circular cross-section of two-convex-refracting surfaces, and is located generally on the optical axis of the eye, i.e., a straight line drawn from the center of the cornea to the macula in the retina at the posterior portion of the globe. Generally, the curvature of the posterior surface of the lens, i.e., the surface adjacent to the vitreous body, is somewhat greater than that of the anterior surface. The lens is closely surrounded by a membranous capsule that serves as an intermediate structure in the support and actuation of the lens. The lens and its capsule are suspended on the optical axis behind the pupil by a circular assembly of many radially-directed collagenous fibers, the zonules, which are attached at their inner ends to the lens capsule and at their outer ends to the ciliary body, a muscular ring of tissue located just within the outer supporting structure of the eye, the sclera. The ciliary body is relaxed in the unaccommodated eye and therefore assumes its largest diameter. According to the classical theory of accommodation, the relatively large diameter of the ciliary body in this condition causes a tension on the zonules which in turn pull radially outward on the lens capsule, causing the equatorial diameter of the lens to increase slightly, and decreasing the anterior-posterior dimension of the lens at the optical axis. Thus, the tension on the lens capsule causes the lens to assume a flattened state wherein the curvature of the anterior surface, and to some extent of the posterior surface, is less than it would be in the absence of the tension. In this state, the refractive power of the lens is relatively low and the eye is focused for clear vision of distant objects, i.e., the unaccommodated state.
In an accommodative state, however, the eye is intended to be focused on a near object, the muscles of the ciliary body contract. This contraction causes the ciliary body to move forward and inward, thereby relaxing the outward pull of the zonules on the equator of the lens capsule. This reduced zonular tension allows the elastic capsule of the lens to contract causing an increase in the anterior-posterior diameter of the lens (i.e., the lens becomes more spherical) resulting in an increase in the optical power of the lens. Because of topographical differences in the thickness of the lens capsule, the central anterior radius of curvature decreases more than the central posterior radius of curvature. This is the accommodated condition of the eye wherein the image of near objects falls sharply on the retina. See Koretz, et al., Scientific American, July, 1988, pages 64-71.
In a simplified model, zonules work like a spring with one end attached to the elastic lens and the other end attached to ciliary muscles. In an unaccommodated state, the spring is pulled taut by the ciliary muscles to force the elastic lens to become thin; in an accommodated state, the spring relaxes to relieve the elastic lens so that it becomes thick (see FIGS. 1a and 1b). In a young and healthy eye, the elastic lens can become thin (low diopter) or thick (high diopter) with about 3 diopters difference. Starting at age mid-forty, a typical eye begins to gradually lose its near distance vision (presbyopia). There are multiple reasons for loss of accommodation. One of them is that the human lens becomes too hard to change back and forth from a thin lens to a thick lens.
The common approach for addressing the problem of the loss of accommodation is to wear reading glasses. However, various attempts have been made to solve presbyopia by implanting an accommodating IOL. A number of U.S. Patents disclose various means for moving the optical body of an IOL along the optical axis anteriorly or posteriorly so that optical power of the IOL can be adjusted to provide either near vision or far vision. For example, Cumming, in his U.S. Pat. Nos. 6,197,059 (issued Mar. 6, 2001) and 5,476,514 (issued Dec. 19, 1995), discloses an accommodating intraocular lens design similar to a plate IOL except that there is a guiding hinge on each side of the haptics (see FIG. 2). The guiding hinge is intended to facilitate the lens in vaulting anteriorly for near vision or posteriorly for distance vision. In other words, the change in lens focus power measured in diopter is achieved by a bi-directional shift of the IOL guided by the hinge, along the optical axis. The more the shifting distance toward the anterior chamber, the more the focus power for the lens. Gwon, in U.S. Pat. No. 6,176,878 (issued Jan. 23, 2001), discloses an accommodating lens design which is adapted to cooperate with the eye to move the optic body bi-directionally, that is anteriorly or posteriorly in the eye. Similarly, Israel, in U.S. Pat. No. 6,013,101 (issued Jan. 11, 2000), discloses another accommodating lens design with haptics in engagement with zonule movement to achieve a bi-directional shift of the lens along the optical axis. In summary, all of these prior art patents utilize various mechanisms to move the optical body of the IOL either anteriorly or posteriorly to achieve near vision, or far vision respectively.
Sarfarazi, in U.S. Pat. No. 5,275,623, issued Jan. 4, 1994, discloses an elliptical accommodating IOL with two optical bodies positioned in the anterior surface and posterior surface respectively. The lens is a closed cell containing fluids. The accommodation is achieved by adjusting the distance between the two optical bodies of the elliptical IOL.
Although accommodating IOL's are known, there is one common complication associated with the IOL implantation: that is posterior chamber opacification (PCO), where cells migrate from equatorial peripheries towards the center of the capsular bag. These cells block incoming light from reaching the retina. Consequently, PCO will cause gradual vision loss, and in some cases complete vision loss, if untreated. This cell migration after cataract surgery is also known as secondary cataract formation.
None of the prior art patents on accommodative lenses includes a feature designed for addressing the PCO problem. In fact, some of the designs disclosed in the prior art may invite the epithelial cells in-growth. For example, FIG. 2 is derived from U.S. Pat. No. 6,197,059, where the IOL is in the backward and forward positions, respectively. The gap between the IOL and posterior surface of the capsular bag in the forward position may invite the growth of epithelial cells so that eventually epithelial cells will occupy any existing space in the capsular bag. Consequently, that structure could lead to secondary cataract formation. Furthermore, the epithelial cell's in-growth into the gap may hinder and eventually prevent the IOL from shifting along the optical axis, thus resulting in a loss in accommodation once again.
It has been estimated that about one-third of the patients who undergo cataract surgery with IOL implantation will eventually develop PCO or secondary cataract formation. The common procedure for treating PCO is using a Yag laser. The laser beam bums cells situated in its pathway to allow images to reach and focus on the retina again so patients regain their vision. This laser treatment is not only a costly procedure but also involves significant risks for the patient. For example, if the laser beam is focused on any part of the eye tissue by mistake, it will cause unnecessary permanent tissue damage or possibly even permanent vision loss.
Accordingly, there is a great need to provide an accommodating lens which can avoid or reduce the possibility of forming a secondary cataract. The present invention discloses a family of accommodating lenses designed to avoid secondary cataract formation while providing accommodating capabilities.